The Productora Cu-Au-Mo deposit is hosted by a Mesozoic hydrothermal breccia complex in the Coastal Cordillara of Region III, northern Chile. Mineralization at Productora extends discontinuously over 8 km in a northeast-oriented corridor. The current resource, which includes the neighboring Alice porphyry Cu-Mo deposit, is estimated at 236.6 Mt grading 0.48% Cu, 0.10 g/t Au, and 135 ppm Mo. The deposit is of significant interest as it presents an example of spatially associated magnetite-apatite mineralization and porphyry-style mineralization. Whilst previous studies have described Productora as an IOCG, we describe the deposit characteristics, based on new information, and present a revised genetic model. Discussion is centered around fluid source and deposit classification as porphyry-related rather than IOCG.
The metallogeny of northern Chile is characterized by orogen-parallel metallogenic belts, each representing discrete epochs of mineralization with stepped eastward younging. During the Mesozoic, a range of deposit styles formed under extensional conditions, including small porphyry Cu, large iron oxide Cu-Au (IOCG), magnetite-apatite, manto-Cu, and precious metal epithermal deposits. Deposits formed under a convergent tectonic regime where subduction style, magmatic arcs and associated back-arc basin stratigraphy, plutonic complexes, and the crusta- scale synarc strike-slip Atacama Fault System (AFS) evolved in response to changes in the prevailing geodynamic conditions.
The Productora deposit is hosted within a thick sequence of broadly coeval rhyolite to rhyodacite lapilli tuffs (128.67 ± 1.29 Ma; U-Pb zircon) and breccias between two major intrusions; the Cachiyuyito tonalite (129.8 ± 0.1 Ma) and Ruta Cinco granodiorite batholith (92.05 ± 1 Ma; U-Pb zircon). Several phases of intermediate and mafic dikes crosscut the deposit. The deposit is situated within a domain of structural complexity defined by major northwest-striking faults (normal oblique slip with dextral strike-slip and northeast-side-up dip-slip) that crosscut major north- to northeast-striking faults (normal oblique-slip with sinistral strike-slip, and east-side-up dip-slip movement).
Hydrothermal breccias, tectonic breccias, veins, and alteration assemblages observed on two cross sections at Productora have been separated into five paragenetic stages. Stage 1 produced quartz-pyrite cemented hydrothermal breccia with muscovite alteration. Stage 2 formed a chaotic matrix-supported tectonic breccia with kaolinite-muscovite-pyrite alteration. Stage 3 tourmaline-pyrite-chalcopyrite ± magnetite ± biotite-cemented hydrothermal breccias are associated with K-feldspar ± albite alteration. Stage 4 veins contain chalcopyrite ± pyrite ± sericite, illite, epidote, and chlorite, and stage 5 veins contain calcite. The stage 3 breccias have been further subdivided into five facies based on their mineralogy. The Productora hydrothermal system crosscuts earlier-formed sodic-calcic alteration and magnetite-apatite mineralization associated with the Cachiyuyito stock.
The breccia complex formed as a result of at least two stages of hydraulic fragmentation (stage 1 and stage 3) and one tectonic breccia event (stage 2), which shows evidence of multiple episodes of reactivation. Alteration minerals are consistent with moderate temperature (<300°–400°C) weakly acidic fluids during stage 1, and moderate- to high-temperature (>300°C) alkaline fluids during stage 3. Lower-temperature (<300°C), weakly acidic fluids prevailed during stages 4B and 4C, and alkaline fluids predominated during stages 4A, 4D, and 5.
Main-stage mineralization is associated with the stage 3 hydrothermal breccia (average grade 0.34–0.62% Cu, 0.8–0.14 g/t Au, 66–128 ppm Mo) and stage 2 tectonic breccia (average grade 0.8% Cu, 0.21 g/t Au, 141 ppm Mo). Chalcopyrite is the dominant hypogene Cu-sulfide mineral and occurs predominantly as stage 3 breccia cement and synbreccia veins with pyrite in equal proportion.
Chalcopyrite and pyrite are disseminated in the stage 2 breccias. Pyrite is elevated in the south compared to the north, and chalcopyrite/pyrite ratios are ~1.00 and <0.25, respectively. Gold and Cu are strongly associated spatially. In this study, mineralization at Productora was dated using Re-Os on stage 3 molybdenite at 130.1 ± 0.6 Ma.
Supergene mineralization in the north of Productora (chrysocolla, malachite, and Cu-wad) is indicative of a geochemically mature weathering environment developed under near neutral to alkaline pH. In the south, the supergene assemblage is less mature (chalcocite/digenite) and indicates in-situ weathering of chalcopyrite.
The Alice Cu-Mo porphyry deposit is situated 400 m to the west of Productora. Mineralization occurs as disseminated chalcopyrite and quartz-pyrite-chalcopyrite ± molybdenite vein stockwork hosted by a granodiorite porphyry stock (121.1 ± 2.1 Ma). Potassic alteration (biotite ± actinolite replacing hornblende) is associated with quartz-sulfide veins. Mineralization was dated by Re-Os on molybdenite at 124.1 ± 0.6 Ma (within error of the porphyry stock). The margins and deeper parts of the system are overprinted by albite ± epidote ± sericite alteration, which locally caused destruction of biotite and chalcopyrite. The Alice porphyry is spatially associated with the Silica Ridge lithocap, which is characterized by massive quartz-altered rock above domains of alunite, pyrophyllite and dickite.
At Productora, δ34S sulphide values range between -8.5 and +2.2 ‰. This is consistent with a magmatic sulfur source and fluids evolving under oxidizing conditions with no significant input from evaporate or seawater-sourced fluids. Stage 3 tourmalines (n = 8) have average initial Sr of 0.70397, consistent with Cretaceous intrusive rocks and mantle-derived Sr.
Based on textural, mineralogical, stable, and radiogenic isotope data, the Productora breccia complex is inferred to be a magmatic-hydrothermal breccia complex formed as a result of explosive volatile fluid release at depths, causing brecciation and alteration of the overlying rock mass. Metal-bearing fluids were of magmatic affinity and evolved under oxidizing conditions. Despite sharing many similarities with the Andean IOCG clan (strong structural control, regional sodic-calcic alteration, local U), fluid evolution at the Productora Cu-Au-Mo deposit is consistent with that of a porphyry magmatic hydrothermal breccia (sulfur-rich, acid alteration assemblages and relatively low magnetite, <5 wt%). The Productora camp provides an excellent example of the close spatial association of Mesozoic magnetite-apatite, porphyry (Alice) and a magmatic-hydrothermal breccia mineralization styles, a relationship seen throughout the Coastal Cordillera of northern Chile.